Implantable medical devices (IMD) may provide a variety of different therapies and other functions including stimulation, drug infusion, physiological sensing, and the like. The IMDs receive programming from an external device and may also share information that has been collected with the external device. Many IMDs communicate with the external device using an inductive form of telemetry where a telemetry head is held in communication range of the IMD so that inductive signals may be exchanged.
The inductive downlink is obtained by a coil within the IMD that is tuned to a telemetry frequency, e.g., 175 kilohertz, being emitted by a coil within the external device. Likewise, the inductive uplink is provided by a coil within the IMD that is tuned to emit signals at a telemetry frequency of the coil of the external device. The uplink and downlink telemetry frequencies are frequently the same and a single coil in each device is tuned to a single frequency that is used for both the uplink and the downlink.
Many IMDs operate on power from a battery, capacitor, or similar power source and therefore have a limited lifetime of operation before a replacement or a recharge is necessary. For IMDs with a rechargeable power source, the recharge energy may be received via an inductive coupling. The external device has a coil tuned to a recharge frequency, e.g., 100 kilohertz, which may differ from the telemetry frequency. Many commercially available IMDs have a second coil that is tuned to the recharge frequency being emitted by the external device.
While using two coils with the IMD adequately establishes telemetry and recharge applications, the size occupied by two separate coils restricts the ability to make smaller IMDs. Thus, miniaturized IMD designs call for a single coil such that the inclusion of the telemetry application precludes inclusion of the recharge application.